GB2065404A

GB2065404A - Voltage selector circuit

Info

Publication number: GB2065404A
Application number: GB7942591A
Authority: GB
Original assignee: Casio Computer Co Ltd
Current assignee: Casio Computer Co Ltd
Priority date: 1979-12-11
Filing date: 1979-12-11
Publication date: 1981-06-24
Also published as: GB2065404B; HK21085A

Abstract

In a voltage selector circuit, a plurality of supply voltages (V0, V1, V2) are respectively supplied to a plurality of switching elements formed of MOS transistors (11, 12, 13, 14, 15) which are turned on and off according to the voltage levels of input signals (A0, A1) supplied thereto to provide a selected one of the supply voltages at output terminal (B0). The substrate electrode of transistor 15 is selectively connected to supply voltages V1, V2 via further switching transistors 17, 18 which are also controlled by signal A0, A1 such that the drain/ substrate diode is never forward biassed, thereby reducing power loss in the circuit. <IMAGE>

Description

SPECIFICATION Voltage selector circuit This invention relates to a voltage selector circuit capable of selectively taking out a plurality of voltages at different levels.

Generally, in driving liquid-crystal display devices for electronic desk top calculator, electronic watches or clocks, etc. according to a dynamic-drive display system, it is necessary to use signals consisting of voltages at 3-value or more levels. Also, in an ink-jet-type printing system, for example, it is required that signals from a character signal generator circuit be D/A-converted as voltage signals at a plurality of levels be successively delivered to deflecting electrodes in accordance with printed characters. In order to obtain such voltages at a plurality of levels, there is used a voltage selector circuit generally so constructed as shown in Fig. 1.The circuit of Fig. 1 is a voltage selector circuit for taking out voltage waveform signals (common electrode driving signals) at 3-value levels to be supplied to a liquid-crystal display device when dynamically driving the display device. In Fig. 1, numerals 1 and 2 designte p-type MOS transistors, while numerals 3, 4 and 5 designate n-type transistors. The bases of the MOS transistors 1 and 4 are supplied with an input signal Ao (32 Hz) as shown in Fig. 2(A), the bases of the MOS transistors 2 and 5 are supplied with an input signal A, (64 Hz) as shown in Fig.

2(B), and the signal A, is applied to the gate of the MOS transistor 3 through an inverter 6.

Further, voltages V0 (O V), V, (- 1.5 V) and V2 (- 3 V) are supplied to the sources of the MOS transistors 1, 5 and 4, respectively. The respective drains of the MOS transistors 2, 3 and 5 are connected together to form an output terminal Bo.

With such construction, when the voltage levels of the input signals Ao and A, are both low, the MOS transistors 1, 2 and 3 are turned on, and the MOS transistors 4 and 5 are turned off, so that the voltage V0 appears at the output terminal Bo. When the voltage levels of the input signals Ao and A, are low and high respectively, the MOS transistors 1 and 5 are turned on, and the MOS transistors 2, 3 and 4 are turned off, so that the voltage V, appears at the output terminal Bo. On the other hand, when the voltage levels of the input signals Ao and A, are high and low respectively, the MOS transistors 2, 3 and 4 are turned on, and the MOS transistors 1 and 5 are turned off, so that the voltage V2 is delivered from the output terminal Bo. By such switching operations of those individual transistors, a voltage waveform signal at 3value levels as shown in Fig. 2(C) may be obtained from the output terminal B,.

Hereupon, the MOS transistor 5 constitutes the point of this circuit. Fig. 3 specifically shows the arrangement of the MOS transistor 5, in which n-type semiconductor layers are formed on a p-type semiconductor substrate at both right and left sides thereof, the substrate being supplied with a substrate voltage C. In this MOS transistor 5, when the voltage at the output terminal B0 is the voltage V2 (- 3 V), the drain electrode is supplied with the voltage V2 (- 3 V), and the voltage V, (- 1.5 V) is applied to the substrate electrode which is supplied with the same voltage as the source voltage. As generally known, a diode characteristic as indicated by broken lines of Fig. 3 is seen between the drain and substrate, so that the voltages are applied in the forward direction in the aforesaid state.

Thus, a diode effect arises to produce a current flow, causing power loss. As a result, the life of batteries used as a power source for some electronic devices, such as electronic wrist watches, will be shortened, requiring more frequency replacement of batteries.

This invention is contrived in consideration of the above-mentioned circumstances, and is intended to provide a voltage selector circuit capable of eliminating power loss caused by the potential different between the voltage of output signal and substrate voltage and ensuring prolonged life of batteries.

In order to attain the above object of the invention, there is provided a voltage selector circuit in which a voltage waveform signal corresponding to one selected among voltages from a plurality of power sources is taken out from a common output terminal by selectively turning on and off switching elements formed of MOS transistors, the circuit being characterized in that corresponding one of the supply voltages is supplied to the substrate electrode of a specified one of the switching elements according to the voltage waveform signal.

As a result, according to the invention, there will never be caused any power loss due to diode effect between the drain and substrate electrodes of the MOS transistor whose drain electrode is connected to the common output terminal. Thus, there may be obtained a voltage selector circuit which may highly effectively be applied to miniature electronic devices or systems using batteries as a working power source, for example.

This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Figure 1 is a circuit diagram of a prior art voltage selector circuit; Figures 2(A) to 2(C) show a time chart of input and output signals as shown in Fig. 1; Figure 3 shows the structure of an n-type MOS transistor; Figure 4 is a circuit diagram of a voltage selector circuit according to an embodiment of this invention; and Figures 5(A) to 5(D) show a time chart of input and output signals and a substrate voitage waveform signal as shown in Fig. 4.

Referring now to the drawings of Figs. 4 and 5(A) to 5(B), an embodiment of this invention will be described in detail.

In Fig. 4, numerals 11 and 1 2 designate ptype MOS transistors, while numerals 13, 14 and 1 5 designate n-type MOS transistors. A signal Ao (32 Hz) is applied to the gates of the MOS transistors 11 and 14, and a signa A, (64 Hz) is applied to the gates of the transistors 1 2 and 1 5 and also supplied to the gate of the transistor 1 3 through an inverter 1 6.

Such construction is just the same as the one shown in Fig. 1. The substrate electrode of the transistor 1 5 is connected with the source of an n-type MOS transistor 1 7 and the drain of an n-type MOS transistor 1 8. The gate and drain of the MOS transistor 1 7 are supplied with the A1 and a voltage V1 respectively.

Further, the gate and source of the transistor 1 8 are supplied with the output signal of the inverter 1 6 and a voltage V2 respectively.

In a voltage selector circuit of the abovementioned construction, the voltage wavelength signal delivered from an output terminal B0 is just the same as the one described in connection with Fig. 1. In the transistor 15, when the voltage levels of the input signals Ao and A, are both low, a voltage V0 (O V) appears at the output terminal B,, and the MOS transistors 1 7 and 1 8 are turned off and on respectively, so that the voltage V2 (- 3 V) is applied to the substrate electrode of the transistor 1 5. When the voltage levels of the input signals Ao and A, are low and high respectively, the voltage V, (- 1.5 V) is delivered from the output terminal B,, and the MOS transistors 1 7 and 1 8 are turned on and off respectively, so that the voltage V, (- 1.5 V) is applied to the substrate electrode of the MOS transistor 1 5. On the other hand, when the voltage levels of the input signals Ao and A1 are high and low respectively, the voltage V2 (- 3 V) is delivered from the output termi- nal B,, and the MOS transistors 1 7 and 18 are turned off and on respectively, so that the voltage V2 (- 3 V) is applied to the substrate electrode of the MOS transistor 1 5. By such switching operations of the MOS transistors 1 7 and 18, the substrate electrode of the MOS transistor 1 5 is supplied with a substrate voltage waveform signal as shown in Fig. 5(D). Waveforms of Figs. 5(A) to 5(C) are identical with the ones shown in Figs. 2(A) to 2(C) respectively.

As may be seen from the substrate voltage waveform of Fig. 5(D), the voltage V2 (- 3 V) lower than the voltage V0 (O V) is applied to the substrate electrode when the output signal B0 is at the voltage VO. Further, the voltage V, (- 1.5 V) or V2 (- 3 V) is applied to the substrate electrode when the output signal B0 is at the voltage V1 or V2, respectively. Thus, the substrate voltage of the MOS transistor 1 5 may be so controlled as to be lower than or equal to the voltage of the output signal B,, so that there will never be caused any power loss due to diode effect between the drain and substrate.

Although the 3-value voltage waveform is obtained with use of the circuit according to the above-mentioned embodiment, the inven- tion is not limited to that case, and may be also applied to case where voltage waveforms including 4-value, 5-value or more levels are obtained. Furthermore, instead of controlling the substrate voltage of an n-type MOS transistor, the substrate voltage of a p-type MOS transistor may be controlled.

Claims

1. Voltage selector circuit in which a voltage waveform signal corresponding to one selected among voltages from a plurality of power sources is taken out from a common output terminal by selectively turning on and off switching elements formed of MOS transistors the circuit being characterized in that corresponding one of the supply voltages is supplied to the substrate electrode of a specified one of the switching elements according to the voltage waveform signal.

2. Voltage selector circuit, substantially as hereinbefore described with reference to the accompanying drawings.